This invention relates to a stent for the vessel mounted in the vessel, such as blood vessel, lymphatic vessel, bile duct or urinary duct to maintain a constant state in the lumen of the vessel.
Heretofore, if a stenosis portion has occurred in the vessel of a living body, in particular the blood vessel, such as artery, a balloon forming portion provided in the vicinity of the distal end of the balloon catheter is inserted into this stenosis portion. This balloon forming portion is expanded to form a balloon to expand the stenosis portion of the blood vessel to improve the blood flow, by way of the transcutaneous blood vessel forming technique (PTA).
It has been clarified that, if the PTA is applied, stenosis tends to be produced at a high probability in the once stenosis portion.
In order to prevent this restenosis, the current practice is to apply a tubular stent in the site processed with the PTA. This stent is inserted into the blood vessel in a diameter-contracted state and subsequently implanted in the blood vessel as it is expanded in diameter to support the blood vessel from its inside to prevent restenosis from being produced in the blood vessel.
As this sort of the stent, there have so far been proposed a balloon expanding stent and a self-expanding stent.
The balloon expanding stent is applied over a balloon provided in a folded and diameter-contracted state in a catheter and, after being inserted in the targeted site for implantation, such as a site of lesion, where the blood vessel is stenosis, the balloon is expanded and increased in diameter to support the inner surface of the blood vessel. Once expanded in diameter, the balloon expanding stent is fixed in this expanded state and cannot be deformed in keeping with the pulsations of the blood vessel wall. On the other hand, if the balloon expanding stent is deformed after being expanded in diameter and implanted in this condition in the blood vessel, it cannot be restored to its original expanded state, such that there is the risk that the stent cannot support the inner surface of the blood vessel reliably.
The self-expanding stent is housed in the diameter-contracted state in a holder, such as a tube, having an outer diameter smaller than the inner diameter of the targeted site for implantation in the blood vessel, and is inserted in the targeted site for implantation in the blood vessel as it is housed in a holder. The stent, thus inserted in the targeted site for implantation in the blood vessel, is extruded or extracted from the holder so as to be expanded in diameter to the pre-contracted state, by exploiting the force of restoration proper to the stent, thus continuing to support the inner wall of the blood vessel.
As this sort of the self-expanding stent, there is proposed such a one obtained on warping a linear member of metal, such as stainless steel, into a sinusoidal or zig-zag design, to form a tube.
With the self-expanding stent formed from a metal linear member, the outer diameter prevailing at the time of expansion is difficult to control precisely, such that the stent is likely to be expanded excessively in comparison with the inner diameter of the blood vessel in which it is implanted. Moreover, if the force of holding the stent in the contracted state is removed, the stent is expanded abruptly. If the stent inserted into the blood vessel is expanded abruptly, the inner wall of the blood vessel is likely to be injured.
As the self-expanding stent, those formed of shape memory alloys, such as Txe2x80x94Ni, Tixe2x80x94Nixe2x80x94Cu or Tixe2x80x94Nixe2x80x94Fe based alloys, have been proposed.
The stent, formed of shape memory alloys, is kept to its size when it is implanted in the targeted loading site in the blood vessel, by the shape memory action, and is subsequently contracted in diameter, so as to be inserted in this diameter-contracted state in the blood vessel. After insertion into the targeted loading site in the blood vessel, this stent is expanded to the size of the shape memory and subsequently exhibits super-elasticity under the body temperature of the living body to continue supporting the inner wall of the blood vessel.
Since the shape memory alloy has extremely high tenacity, such that it exerts an extremely large mechanical pressure to a portion of the inner wall of the blood vessel, thus possibly damaging the blood vessel. Moreover, there are occasions wherein the stent formed of a shape memory alloy is not uniformly expanded in diameter against the inner wall of the blood vessel when implanted in the blood vessel. If a portion of the stent compresses against the inner wall of the blood vessel prematurely to commence to be expanded in diameter, the blood vessel cannot be expanded uniformly. In this case, the portion of the blood vessel, against which a portion of the stent has compressed prematurely, is enlarged excessively in diameter, and hence is likely to be damaged.
The stent formed of metal such as shape memory alloy, once implanted in the vessel, such as blood vessel, is permanently left in the living body unless it is taken out by surgical operations.
It is an object of the present invention to provide a stent for a vessel, such as blood vessel, which is able to keep the vessel in the expanded state reliably without injuring the vessel.
It is another object of the present invention to provide a stent for a vessel which disappears after lapse of a pre-set period after implantation in the vessel to eliminate the necessity of executing a surgical operation of taking out the stent from the vessel after restoration of the site of lesion.
It is another object of the present invention to provide a stent for a vessel which is able to support the vessel, such as blood vessel, with a uniform force.
It is yet another object of the present invention to provide a stent for a vessel which can be inserted into a meandering vessel, such as blood vessel, with good trackability, and which can be easily and reliably implanted in the targeted site in the vessel.
For accomplishing the above object, the present invention provides a stent for a vessel implanted in the vessel of the living body including a main body portion of the stent formed into a tube by a yarn, which is formed of a biodegradable polymer, exhibiting a shape memory function. The main body portion of the stent is shape-memorized to a size that can be retained in the vessel. The main body portion of the stent is implanted in the vessel of the living body as it is contracted in diameter by an external force, and is enlarged in diameter by being heated with the body temperature of the living body.
The yarn used is a concatenated continuous monofilament yarn or a multi-filament yarn made up of a plurality of monofilament yarns unified together.
The main body portion of the stent is formed by the yarn formed of a biodegradable polymer being wound to a tube as the yarn is bent in a zigzag design and is enlarged or contracted in diameter with the bends of the yarn as displacing portions.
In the main body portion of the stent, at least part of neighboring bends of the yarns wound to a tube as the yarns are bent in a zigzag design are connected to one another so that a pre-set tubular shape of the main body portion of the stent is positively maintained on contracting or enlarging its diameter.
The tubular main body portion of the stent is formed by arraying plural yarns each connected to form a ring as each yarn is bent in a zigzag design, these yarns being juxtaposed along the axial direction of the main body portion of the stent to form a tube.
Each yarn making up the main body portion of the stent is formed of a biodegradable polymer having the glass transition temperature not higher than approximately 70xc2x0 C. Thus, the main body portion of the stent is enlarged in diameter to its shape-memorized state at a temperature close to the body temperature.
Each yarn making up the main body portion of the stent is formed of a biodegradable polymer compounded from one or more of polylactic acid (PLLA), polyglycolic acid (PGA), a copolymer of polyglycolic acid and polylactic acid, polydioxanone, a copolymer of trimethylene carbonate and glycolid, and a copolymer of polyglycolic acid or polylactic acid and xcex5-caprolactone.
If an radiopaque medium is mixed into or deposited on the yarn, the state of implantation of the stent in the vessel can be easily checked from outside the living body using X-rays.
If antithrombotic drugs or drugs for suppressing neointimal formation are mixed into or deposited on the yarn formed by the biodegradable polymer, these drugs can be administered in a sustained fashion as the stent is dissolved.
Moreover, if a radiation source radiating xcex2-rays or a radiation source radiating xcex3-rays is mixed into or deposited on the yarn formed of the biodegradable polymer, these rays can be radiated to the lesion as the stent is inserted into the living body, thus assuring sustained irradiation of radiation rays.
Other objects and advantages of the present invention will become apparent from the following description which is made with reference to the accompanying drawings.